There are several methods in the prior art for transmitting video over limited bandwidth communication paths. One prior art method stored a video picture and then slowly transmitted the video as FM data. The FM data was transmitted using ham radio equipment or dial-up telephone lines operating using audio tones usually from 600 hertz to 2400 hertz. This allowed most video pictures to be sent in approximately 8 to 30 second with resolutions of approximately 128.times.128 pixels to 256.times.256 pixels respectively. The picture usually had a 6 bit gray scale, i.e. 64 shades of gray. The picture information was loaded into memory at the transmitter end, at a live video rate, from a standard video source. Once stored, the information was accessed from memory at a much slower rate and in turn fed to a circuit which converted the information into FM data, on the basis of the higher the digital count the higher the frequency. The video information was combined with sync information to signal the receiver unit when a line of information was completed and when a picture was completed. Because complete pictures were sent and video pictures were made up of shades of gray, the FM method worked satisfactorily. If the carrier was slightly distorted or off frequency, it only shifted the gray scale and was basically unnoticeable in the transmitted picture. A memory device was used at each end. The memory took various forms but was typically a memory chip or chips. The output of the memory was either displayed on a built in cathode ray tube (CRT) or was coupled to an external CRT display. As new pictures were sent, the new picture information would scroll down the screen replacing the old information in memory as it was received.
The analog data was found to be advantageous because given any bandwidth and time length, analog data could carry more information than digital data. Consequently, relative to sending video data that has not been compressed, the FM method was considered fast. However, since the systems had been designed to work over the switched telephone network, the frequencies were limited to below 3,000 hertz. In view of this frequency limitation, the FM method became disadvantageous and the digital system became more advantageous with the increasing speed of microprocessors.
The advent of high speed modems and more powerful microprocessors enabled digital systems to develop rapidly. These digital systems use a different method of transmission than the FM transmission method. The digital systems send an initial picture to the receiver unit and then update only the changes in video. Because of the high speed processors and complex algorithms, they are able to get more picture updates over the switched telephone network than is possible using an FM method. For example, they send an initial picture to the receiver and then they update only the changes. Because most still camera pictures do not change significantly, the digital method can give an appearance of providing fast video. Also, compression algorithms are dynamic and can sacrifice resolution for speed if desired.
However, digital transmission has its drawbacks. One drawback is that with large changes in the video picture, the video picture can be slowed down substantially. And while the cost of computers seems to be dropping continuously, computers are still significantly more expensive than the cost of an FM system. We have discovered a method and apparatus for transmitting video as FM data while at the same time obtaining relatively high speed video.
We have found that in order to obtain higher speed video, it is advantageous to avoid the switched telephone network, and to instead use direct copper wire or leased copper line. By avoiding the switched telephone network, much higher frequencies can be transmitted down the wire, allowing substantially greater speed using the FM method. In this manner, updates of several times per second are possible.
It has been found, however, that when a higher speed video is transmitted using a system similar to a system in the prior art, a picture is produced which may be visually annoying. It is, therefore, an object of the invention to provide a method and apparatus for transmitting video using an FM transmission system but obtaining a substantially greater speed than in the prior art.
Another object of the present invention is to transmit video using an FM transmission system while providing for a smooth display of the incoming video picture and alleviating the production of a picture which is visually annoying.
A further object of the present invention is to provide an FM transmission method for transmitting video that is simple and economical in construction and operation.
Other objects and advantages of the present invention will become apparent as the description proceeds.